RFC 5204 HIP Rendezvous Extension April 20081. Introduction
The Host Identity Protocol (HIP) Architecture [RFC4423] introduces
the rendezvous mechanism to help a HIP node to contact a frequently
moving HIP node. The rendezvous mechanism involves a third party,
the rendezvous server (RVS), which serves as an initial contact point
("rendezvous point") for its clients. The clients of an RVS are HIP
nodes that use the HIP Registration Extension [RFC5203] to register
their HIT->IP address mappings with the RVS. After this
registration, other HIP nodes can initiate a base exchange using the
IP address of the RVS instead of the current IP address of the node
they attempt to contact. Essentially, the clients of an RVS become
reachable at the RVS's IP address. Peers can initiate a HIP base
exchange with the IP address of the RVS, which will relay this
initial communication such that the base exchange may successfully
complete.
2. Terminology
This section defines terms used throughout the remainder of this
specification.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
In addition to the terminology defined in the HIP specification
[RFC5201] and the HIP Registration Extension [RFC5203], this document
defines and uses the following terms:
Rendezvous Service
A HIP service provided by a rendezvous server to its rendezvous
clients. The rendezvous server offers to relay some of the
arriving base exchange packets between the initiator and
responder.
Rendezvous Server (RVS)
A HIP registrar providing rendezvous service.
Rendezvous Client
A HIP requester that has registered for rendezvous service at a
rendezvous server.
Rendezvous Registration
A HIP registration for rendezvous service, established between a
rendezvous server and a rendezvous client.
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RFC 5204 HIP Rendezvous Extension April 20083. Overview of Rendezvous Server Operation
Figure 1 shows a simple HIP base exchange without a rendezvous
server, in which the initiator initiates the exchange directly with
the responder by sending an I1 packet to the responder's IP address,
as per the HIP specification [RFC5201].
+-----+ +-----+
| |-------I1------>| |
| I |<------R1-------| R |
| |-------I2------>| |
| |<------R2-------| |
+-----+ +-----+
Figure 1: HIP base exchange without rendezvous server.
The End-Host Mobility and Multihoming with the Host Identity Protocol
specification [RFC5206] allows a HIP node to notify its peers about
changes in its set of IP addresses. This specification presumes
initial reachability of the two nodes with respect to each other.
However, such a HIP node MAY also want to be reachable to other
future correspondent peers that are unaware of its location change.
The HIP Architecture [RFC4423] introduces rendezvous servers with
whom a HIP node MAY register its host identity tags (HITs) and
current IP addresses. An RVS relays HIP packets arriving for these
HITs to the node's registered IP addresses. When a HIP node has
registered with an RVS, it SHOULD record the IP address of its RVS in
its DNS record, using the HIP DNS resource record type defined in the
HIP DNS Extension [RFC5205].
+-----+
+--I1--->| RVS |---I1--+
| +-----+ |
| v
+-----+ +-----+
| |<------R1-------| |
| I |-------I2------>| R |
| |<------R2-------| |
+-----+ +-----+
Figure 2: HIP base exchange with a rendezvous server.
Figure 2 shows a HIP base exchange involving a rendezvous server. It
is assumed that HIP node R previously registered its HITs and current
IP addresses with the RVS, using the HIP Registration Extension
[RFC5203]. When the initiator I tries to establish contact with the
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RFC 5204 HIP Rendezvous Extension April 2008
responder R, it must send the I1 of the base exchange either to one
of R's IP addresses (if known via DNS or other means) or to one of
R's rendezvous servers. Here, I obtains the IP address of R's
rendezvous server from R's DNS record and then sends the I1 packet of
the HIP base exchange to RVS. RVS, noticing that the HIT contained
in the arriving I1 packet is not one of its own, MUST check its
current registrations to determine if it needs to relay the packets.
Here, it determines that the HIT belongs to R and then relays the I1
packet to the registered IP address. R then completes the base
exchange without further assistance from RVS by sending an R1
directly to the I's IP address, as obtained from the I1 packet. In
this specification, the client of the RVS is always the responder.
However, there might be reasons to allow a client to initiate a base
exchange through its own RVS, like NAT and firewall traversal. This
specification does not address such scenarios, which should be
specified in other documents.
3.1. Diagram Notation
Notation Significance
-------- ------------
I, R I and R are the respective source and destination IP
addresses in the IP header.
HIT-I, HIT-R HIT-I and HIT-R are the initiator's and the
responder's HITs in the packet, respectively.
REG_REQ A REG_REQUEST parameter is present in the HIP header.
REG_RES A REG_RESPONSE parameter is present in the HIP header.
FROM:I A FROM parameter containing the IP address I is
present in the HIP header.
RVS_HMAC An RVS_HMAC parameter containing an HMAC keyed with the
appropriate registration key is present in the HIP
header.
VIA:RVS A VIA_RVS parameter containing the IP address RVS of a
rendezvous server is present in the HIP header.
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RFC 5204 HIP Rendezvous Extension April 20083.2. Rendezvous Client Registration
Before a rendezvous server starts to relay HIP packets to a
rendezvous client, the rendezvous client needs to register with it to
receive rendezvous service by using the HIP Registration Extension
[RFC5203] as illustrated in the following schema:
+-----+ +-----+
| | I1 | |
| |--------------------------->| |
| |<---------------------------| |
| I | R1(REG_INFO) | RVS |
| | I2(REG_REQ) | |
| |--------------------------->| |
| |<---------------------------| |
| | R2(REG_RES) | |
+-----+ +-----+
Rendezvous client registering with a rendezvous server.
3.3. Relaying the Base Exchange
If a HIP node and one of its rendezvous servers have a rendezvous
registration, the rendezvous servers relay inbound I1 packets (that
contain one of the client's HITs) by rewriting the IP header. They
replace the destination IP address of the I1 packet with one of the
IP addresses of the owner of the HIT, i.e., the rendezvous client.
They MUST also recompute the IP checksum accordingly.
Because of egress filtering on the path from the RVS to the client
[RFC2827][RFC3013], a HIP rendezvous server SHOULD replace the source
IP address, i.e., the IP address of I, with one of its own IP
addresses. The replacement IP address SHOULD be chosen according to
relevant IPv4 and IPv6 specifications [RFC1122][RFC3484]. Because
this replacement conceals the initiator's IP address, the RVS MUST
append a FROM parameter containing the original source IP address of
the packet. This FROM parameter MUST be integrity protected by an
RVS_HMAC keyed with the corresponding rendezvous registration
integrity key [RFC5203].
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RFC 5204 HIP Rendezvous Extension April 2008
I1(RVS, R, HIT-I, HIT-R
I1(I, RVS, HIT-I, HIT-R) +---------+ FROM:I, RVS_HMAC)
+----------------------->| |--------------------+
| | RVS | |
| | | |
| +---------+ |
| V
+-----+ R1(R, I, HIT-R, HIT-I, VIA:RVS) +-----+
| |<---------------------------------------------| |
| | | |
| I | I2(I, R, HIT-I, HIT-R) | R |
| |--------------------------------------------->| |
| |<---------------------------------------------| |
+-----+ R2(R, I, HIT-R, HIT-I) +-----+
Rendezvous server rewriting IP addresses.
This modification of HIP packets at a rendezvous server can be
problematic because the HIP protocol uses integrity checks. Because
the I1 does not include HMAC or SIGNATURE parameters, these two end-
to-end integrity checks are unaffected by the operation of rendezvous
servers.
The RVS SHOULD verify the checksum field of an I1 packet before doing
any modifications. After modification, it MUST recompute the
checksum field using the updated HIP header, which possibly included
new FROM and RVS_HMAC parameters, and a pseudo-header containing the
updated source and destination IP addresses. This enables the
responder to validate the checksum of the I1 packet "as is", without
having to parse any FROM parameters.
4. Rendezvous Server Extensions
This section describes extensions to the HIP Registration Extension
[RFC5203], allowing a HIP node to register with a rendezvous server
for rendezvous service and notify the RVS aware of changes to its
current location. It also describes an extension to the HIP
specification [RFC5201] itself, allowing establishment of HIP
associations via one or more HIP rendezvous server(s).
4.1. RENDEZVOUS Registration Type
This specification defines an additional registration for the HIP
Registration Extension [RFC5203] that allows registering with a
rendezvous server for rendezvous service.
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RFC 5204 HIP Rendezvous Extension April 2008
Number Registration Type
------ -----------------
1 RENDEZVOUS
4.2. Parameter Formats and Processing4.2.1. RVS_HMAC Parameter
The RVS_HMAC is a non-critical parameter whose only difference with
the HMAC parameter defined in the HIP specification [RFC5201] is its
"type" code. This change causes it to be located after the FROM
parameter (as opposed to the HMAC):
Type 65500
Length Variable. Length in octets, excluding Type, Length, and
Padding.
HMAC HMAC computed over the HIP packet, excluding the
RVS_HMAC parameter and any following parameters. The
HMAC is keyed with the appropriate HIP integrity key
(HIP-lg or HIP-gl) established when rendezvous
registration happened. The HIP "checksum" field MUST be set
to zero, and the HIP header length in the HIP common header
MUST be calculated not to cover any excluded parameter
when the HMAC is calculated. The size of the
HMAC is the natural size of the hash computation
output depending on the used hash function.
To allow a rendezvous client and its RVS to verify the integrity of
packets flowing between them, both SHOULD protect packets with an
added RVS_HMAC parameter keyed with the HIP-lg or HIP-gl integrity
key established while registration occurred. A valid RVS_HMAC SHOULD
be present on every packet flowing between a client and a server and
MUST be present when a FROM parameter is processed.
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RFC 5204 HIP Rendezvous Extension April 20084.2.3. VIA_RVS Parameter
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 65502
Length Variable
Address An IPv6 address or an IPv4-in-IPv6 format IPv4 address.
After the responder receives a relayed I1 packet, it can begin to
send HIP packets addressed to the initiator's IP address, without
further assistance from an RVS. For debugging purposes, it MAY
include a subset of the IP addresses of its RVSs in some of these
packets. When a responder does so, it MUST append a newly created
VIA_RVS parameter at the end of the HIP packet. The main goal of
using the VIA_RVS parameter is to allow operators to diagnose
possible issues encountered while establishing a HIP association via
an RVS.
4.3. Modified Packets Processing
The following subsections describe the differences of processing of
I1 and R1 while a rendezvous server is involved in the base exchange.
4.3.1. Processing Outgoing I1 Packets
An initiator SHOULD NOT send an opportunistic I1 with a NULL
destination HIT to an IP address that is known to be a rendezvous
server address, unless it wants to establish a HIP association with
the rendezvous server itself and does not know its HIT.
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RFC 5204 HIP Rendezvous Extension April 2008
When an RVS rewrites the source IP address of an I1 packet due to
egress filtering, it MUST add a FROM parameter to the I1 that
contains the initiator's source IP address. This FROM parameter MUST
be protected by an RVS_HMAC keyed with the integrity key established
at rendezvous registration.
4.3.2. Processing Incoming I1 Packets
When a rendezvous server receives an I1 whose destination HIT is not
its own, it consults its registration database to find a registration
for the rendezvous service established by the HIT owner. If it finds
an appropriate registration, it relays the packet to the registered
IP address. If it does not find an appropriate registration, it
drops the packet.
A rendezvous server SHOULD interpret any incoming opportunistic I1
(i.e., an I1 with a NULL destination HIT) as an I1 addressed to
itself and SHOULD NOT attempt to relay it to one of its clients.
When a rendezvous client receives an I1, it MUST validate any present
RVS_HMAC parameter. If the RVS_HMAC cannot be verified, the packet
SHOULD be dropped. If the RVS_HMAC cannot be verified and a FROM
parameter is present, the packet MUST be dropped.
A rendezvous client acting as responder SHOULD drop opportunistic I1s
that include a FROM parameter, because this indicates that the I1 has
been relayed.
4.3.3. Processing Outgoing R1 Packets
When a responder replies to an I1 relayed via an RVS, it MUST append
to the regular R1 header a VIA_RVS parameter containing the IP
addresses of the traversed RVSs.
4.3.4. Processing Incoming R1 Packets
The HIP specification [RFC5201] mandates that a system receiving an
R1 MUST first check to see if it has sent an I1 to the originator of
the R1 (i.e., the system is in state I1-SENT). When the R1 is
replying to a relayed I1, this check SHOULD be based on HITs only.
In case the IP addresses are also checked, then the source IP address
MUST be checked against the IP address included in the VIA_RVS
parameter.
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RFC 5204 HIP Rendezvous Extension April 20085. Security Considerations
This section discusses the known threats introduced by these HIP
extensions and the implications on the overall security of HIP. In
particular, it argues that the extensions described in this document
do not introduce additional threats to the Host Identity Protocol.
It is difficult to encompass the whole scope of threats introduced by
rendezvous servers because their presence has implications both at
the IP and HIP layers. In particular, these extensions might allow
for redirection, amplification, and reflection attacks at the IP
layer, as well as attacks on the HIP layer itself, for example, man-
in-the-middle attacks against the HIP base exchange.
If an initiator has a priori knowledge of the responder's host
identity when it first contacts the responder via an RVS, it has a
means to verify the signatures in the HIP base exchange, which
protects against man-in-the-middle attacks.
If an initiator does not have a priori knowledge of the responder's
host identity (so-called "opportunistic initiators"), it is almost
impossible to defend the HIP exchange against these attacks, because
the public keys exchanged cannot be authenticated. The only approach
would be to mitigate hijacking threats on HIP state by requiring an
R1 answering an opportunistic I1 to come from the same IP address
that originally sent the I1. This procedure retains a level of
security that is equivalent to what exists in the Internet today.
However, for reasons of simplicity, this specification does not allow
the establishment of a HIP association via a rendezvous server in an
opportunistic manner.
6. IANA Considerations
This section is to be interpreted according to the Guidelines for
Writing an IANA Considerations Section in RFCs [RFC2434].
This document updates the IANA Registry for HIP Parameters Types by
assigning new HIP Parameter Types values for the new HIP Parameters
defined in Section 4.2:
o RVS_HMAC (defined in Section 4.2.1)
o FROM (defined in Section 4.2.2)
o VIA_RVS (defined in Section 4.2.3)
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RFC 5204 HIP Rendezvous Extension April 2008
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